Multi-layer circuit devices such as micro-fluid ejection heads have a plurality of electrically conductive layers separated by insulating dielectric layers and applied adjacent to a substrate. Thermal energy generators or heating elements, usually resistors, are located on a surface of the substrate to heat and vaporize the fluid to be ejected.
Conventionally, the substrate material has been silicon, and the heads have been fabricated on typically round single crystalline silicon wafers. Silicon has favorable thermal conductivities such that heat is rapidly dissipated from the heater region. Silicon is also capable of accepting (or being polished to) a smooth finish, which is desirable for predictable and consistent bubble nucleation. However, the use of silicon substrates has proved unsuitable in achieving micro-fluid ejection heads, such as ink jet heads, having a relatively wide swath from a single piece of silicon. For example, silicon wafers used to make silicon chips are available only in round format because the basic manufacturing process is based on a single seed crystal that is rotated in a high temperature crucible to produce a cylindrical ingot that is processed into thin wafers for the semiconductor industry. The circular wafer stock is very efficient when the micro-fluid ejection head chip dimensions are small relative to the diameter of the wafer. However, such circular wafer stock is inherently inefficient for use in making large rectangular silicon chips such as chips having a dimension of 2.5 centimeters or greater. In fact the expected yield of silicon chips having a dimension of greater than 2.5 centimeters from a 6″ circular wafer is typically less than about 20 chips. Such a low chip yield per wafer makes the cost per chip prohibitively expensive. In addition, with respect to at least micro-fluid ejector heads, much of the silicon “real estate” has traditionally been used for device (e.g., transistor/logic) fabrication. Conventional fabrication processes and wafers have at least some inherent defect density of defects (e.g., impurity concentrations/lattice defects), any of which might cause a device (e.g., a transistor) to fail, thereby effecting the performance and/or usability of the entire head containing that device. For example, if there are 100 chips on a wafer and 7 such defects, odds are that 6-7 chips will be lost in this fashion, representing a ˜7% yield loss. Accordingly, if there are only 10 chips on the wafer and 6-7 are lost, the impact would be much higher (e.g., 60-70%).
Accordingly, there is a need for improved structures and methods for making micro-fluid ejection heads, particularly ejection heads suitable for ejection devices having an ejection swath dimension of greater than about 2.5 centimeters.
In this regard, it has been discovered that substrates for providing micro-fluid ejection heads having a relatively wide swath may be made by utilizing non-conventional substrate materials including, but not limited to, glass, ceramic, metal, and plastic materials. While ceramic materials such as alumina, silicon nitride, and beryllia have adequate thermal conductivity properties, other ceramic and glass materials, such as glass and low temperature co-fired ceramic (LTCC) substrates (which have a significant glass fraction that can be 50% or more) have relatively low thermal conductivities and are unable to effectively dissipate enough heat to prevent overheating of the head, especially if the ejection head is operated at a high frequency. This inability to effectively dissipate heat can undesirably affect performance of the head. For example, fluid, such as ink, entering the thermal ejector region after a fluid ejection phase may boil due to the high temperature in the thermal ejector region. Effective heat dissipation after a fluid ejection phase avoids such conditions.
Another disadvantage of alumina and other ceramic substrates is that it is at best expensive and very technically challenging to achieve the extremely smooth finish which is required for predictable and consistent bubble nucleation. For example, it has been observed that a surface roughness of greater than about 75 Å average roughness (Å Ra) can contribute to unpredictable and inconsistent bubble nucleation and disadvantageously affect fluid ejection.
Exemplary embodiments provided in the present disclosure advantageously provide for the manufacture of ceramic substrates having suitable thermal conductivity and smoothness properties to achieve predictable and consistent fluid bubble so as to be suitable for providing micro-fluid ejection heads.
An advantage of the exemplary heads and methods described herein is that, for example, large array substrates may be fabricated from non-conventional substrate materials including, but not limited to, glass, ceramic, metal, and plastic materials. The term “large array” as used herein means that the substrate is a unitary substrate having a dimension in one direction of greater than about 2.5 centimeters. However, the heads and methods described herein may also be used for conventional size ejection head substrates.
Accordingly, in one aspect, methods are provided for fabricating micro-fluid ejection heads. In one embodiment, such a method involves substantially flattening a surface of a substrate to substantially remove a camber; applying a first glass material adjacent to the substantially flattened surface; applying a second glass layer adjacent to the first glass layer, wherein the second glass layer has a surface roughness of no greater than about 75 Å Ra; and forming thermal fluid ejection actuators adjacent (e.g., on the free surface of) to the second glass layer.
In another embodiment, a method for fabricating micro-fluid ejection heads involves substantially flattening a surface of a substrate to substantially remove a camber; polishing the flattened substrate to provide a surface having a predetermined peak roughness; applying a first glass material adjacent to the polished flattened substrate at a thickness at least as thick as the peak roughness to provide a first glass layer, applying a second glass layer adjacent to the first glass layer, wherein the second glass layer has a surface roughness of no greater than about 75 Å Ra; and forming thermal fluid ejection actuators adjacent to the second glass layer.
Still another embodiment is provided involving a micro-fluid ejection head having a substrate with first and second glass layers disposed adjacent to a surface thereof and a plurality of fluid ejection actuators disposed adjacent to the second glass layer. The first glass layer is thicker than the second glass layer and the second glass layer has a surface roughness of no greater than about 75 Å Ra.